The present invention relates to an arrangement and a method for efficient combustion of fuel in a combustion engine by controlled injection of the fuel.
The type of combustion engines commonly known as HCCI (Homogenous Charge Compression Ignition) engines may be regarded as a combination of an Otto engine and a diesel engine. In HCCI engines, fuel and air are mixed in a combustion space to form a homogeneous lean mixture. The fuel mixture is compressed until it is self-ignited by compression heat. In this situation, the combustion takes place with a so-called premixed flame, resulting in very rapid combustion at a relatively low combustion temperature, leading to low contents of nitrogen oxides NOx in the exhaust gases. The fact that the fuel mixture is lean also results in low contents of soot particles in the exhaust gases. HCCI engines have the disadvantage that the respective components are subject to relatively heavy loads and that such rapid combustion causes considerable noise. Another disadvantage of HCCI engines is that it is difficult to control the self-ignition so that it takes place at an optimum crankshaft angle.
In diesel engines, fuel is injected into a combustion space when the piston is about to complete the air compression stroke. At that stage the compressed air is at such a high temperature that the fuel self-ignites substantially immediately when it is injected into the combustion space. It is therefore relatively easy to control the combustion process so that it takes place at an optimum crankshaft angle. However, the fuel and the air are not premixed before the self-ignition of the fuel takes place. The combustion therefore takes place with a so-called diffusion flame. In a diffusion flame, the combustion takes place locally where the oxygen in the air and the fuel diffuse (mix) together. The combustion process is thus lengthened. The advantage of such a combustion process is that the respective components are not subject to the same mechanical stresses as in rapid combustion. However, a diffusion flame results in a combustion process which at least initially takes place with a fatter than stoichiometric mixture ratio, which may cause the formation of relatively high contents of soot particles in exhaust gases from diesel engines. Combustion with a diffusion flame is a longer combustion process and high combustion temperatures occur during parts of the process. Diesel engines therefore have relatively high contents of nitrogen oxides in their exhaust gases.
U.S. Pat. No. 6,820,587 refers to a method for controlling the injection of fuel in a combustion engine whereby the fuel is injected at a very high pressure in a combustion space. The fuel is thus at such a high initial velocity that it does not immediately react with the oxygen in the combustion space, making it possible for a larger amount of air to mix with the fuel before combustion than in a conventional diesel engine. The improved mixture of air and fuel results in combustion with a diffusion flame which has a leaner fuel mixture, resulting in a smaller amount of soot particles in the exhaust gases. The soot particles formed also burn to a certain extent when they pass through the hottest region of the diffusion flame, in which the combustion takes place at a stoichiometric ratio.